Cast irons for high temperature use

ABSTRACT

Cast irons are provided which have no intentional addition of molybdenum (Mo) and which have compositions with additional silicon that provide certain advantages, such as lower cost, higher use temperatures, and increased tensile strength, over conventional Mo-bearing cast irons. In the cast irons, Mo is absent or present only as an impurity element.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with Government support under Agreement No. ERD-96-XJ190 awarded by the Department of Energy. The Government may have certain rights in the invention.

TECHNICAL FIELD

The present invention relates to cast irons and more particularly to certain cast irons which have no intentional addition of Mo and which have compositions to provide certain cost and use advantages.

BACKGROUND OF THE INVENTION

Cast irons are used for exhaust manifold and other automotive component applications. The high-temperature cast irons used for exhaust manifolds, which operate at 650 to 700 degrees C. and above, contain alloying elements such as C, Si, and Mo. The element Mo (molybdenum) is used only up to a maximum of 1% by weight.

The worldwide demand of Mo has sharply increased the price of Mo (approximately 10 fold). Even with only up to 1% by weight of Mo in cast irons for exhaust manifolds, the 10-fold cost increase can add substantial additional cost in connection with continued use of existing cast irons.

The increased cost of Mo has posed a need for cast irons that are substantially free of Mo; i.e., for cast irons that do not include an intentional addition of Mo as an alloying element.

SUMMARY OF THE INVENTION

The present invention provides certain cast irons which have no intentional addition of Mo and which have compositions to provide certain cost and use advantages over conventional Mo-bearing cast irons. These advantages include, but are not limited to, lower cost and a higher ferrite-to-austenite transformation temperature resulting in higher use temperatures than conventional Mo-bearing cast irons.

In practice of an embodiment of the invention, a cast iron is provided wherein Si is present in an amount of about 3.5 weight % to about 6.0 weight % and wherein Mo is absent or present as an impurity element.

In an illustrative embodiment of the invention, a cast iron is provided consisting essentially of, in weight %, about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, and balance Fe wherein Mo is absent, such as substantially 0 weight %, or present as an impurity element, such as not exceeding about 0.01 weight %. Si preferably is about 4.5% to about 6.0% by weight and even more preferably about 4.75% to about 5.5%. C preferably is about 2.5% to about 3.5% of the cast iron composition.

In a further illustrative embodiment of the invention, a nodular cast iron is provided having a composition, in weight %, consisting essentially of about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, up to about 2.0% Cr, up to about 2.0% Mn, up to about 0.75% Ni, up to about 2.5% W, up to about 2.5% Cu, up to about 1.0% V up to about 1.0% Ti, an effective amount of a nodularizing agent, and balance Fe wherein Mo is absent or present as an impurity element.

One or more of Cu, W, V, or Ti can be present in the cast iron composition in a respective amount of about 0.25% to about 2.5% for Cu, about 0.25% to about 2.5% for W, about 0.025% to about 0.2% for V, or about 0.1% to about 0.25% for Ti.

Cast irons pursuant to the invention that are free of Mo or have only impurity Mo are advantageous to provide a higher ferrite-to-austenite transformation temperature (e.g. AC₁ temperature) that is about 50 degrees C. higher than a conventional Mo-containing cast iron, although the microstructure is similar in its graphite content to that of Mo-containing cast iron. For example, cast irons of the invention can exhibit an AC₁ temperature of about 880 degrees C. and above. Cast irons of the invention can operate at temperatures of 650-700 degrees C. and above, such as 750 degrees C., encountered in exhaust manifold applications.

Moreover, the cast irons of the invention are advantageous to provide a higher tensile strength at room temperature and above (e.g. at 600 degrees C.) than a conventional Mo-containing cast iron. For example, the cast irons of the invention can have an ultimate tensile strength above 90 ksi at 23 degrees C. (room temperature-RT) and an ultimate tensile strength above 30 ksi at 600 degrees C. These advantages are achieved at lower alloy cost since no intentional Mo addition is made to the cast iron.

These and other advantages of the present invention will become more readily apparent form the following detailed description taken with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of phase stability in weight % as a function of temperature for comparison cast iron GM-1.

FIG. 2 is a graph of phase content in weight % other than ferrite and austenite as a function of temperature for comparison cast iron GM-1.

FIG. 3 is a graph of phase stability in weight % as a function of temperature for comparison cast iron GM-2.

FIG. 4 is a graph of phase content in weight % other than ferrite and austenite as a function of temperature for comparison cast iron GM-2.

FIG. 5 is a graph of phase stability in weight % as a function of temperature for evaluated cast iron GMO-1.

FIG. 6 is a graph of phase content in weight % other than ferrite and austenite as a function of temperature for evaluated cast iron GMO-1.

FIG. 7 contains Table 8.

FIG. 8 is a DSC (differential scanning calorimetry) showing transformation temperatures for comparison cast iron GM-2 and evaluated cast irons GMO-10 and GMO-10A.

FIGS. 9 a and 9 b are photomicrographs of the cast microstructures of evaluated cast irons GMO-10 and GMO-10A, respectively, with a 2% Nital etch.

FIGS. 10 a and 10 b are photomicrographs of the cast microstructures of comparison cast irons GM-1 and GM-2, respectively, with a 2% Nital etch.

FIG. 11 is a graph comparing 0.2% yield strength versus temperature of comparison cast irons GM-1 and GM-2 from different casting lots and evaluated cast irons GMO-10 and GMO-10A.

FIG. 12 is a graph comparing ultimate tensile strength (UTS) versus temperature of comparison cast irons GM-1 and GM-2 from different casting lots and evaluated cast irons GMO-10 and GMO-10A.

FIG. 13 is a DSC showing transformation temperatures for evaluated cast irons GMO-21A, -21B, -22A, and -22B.

FIG. 14 is a bar graph comparing 0.2% yield strength of comparison cast iron GM-2 and GMO-10, -10A, 21A, -21B, -22A, and -22B at RT.

FIG. 15 is a bar graph comparing of ultimate tensile strength of comparison cast iron GM-2 and GMO-10, -10A, 21A, -21B, -22A, and -22B at RT.

FIG. 16 is a bar graph comparing 0.2% yield strength of comparison cast iron GM-2 and GMO-10, -10A, 21A, -21B, -22A, and -22B at 600 degrees C.

FIG. 17 is a bar graph comparing ultimate tensile strength of comparison cast iron GM-2 and GMO-10, -10A, 21A, -21B, -22A, and -22B at 600 degrees C.

FIG. 18 is a bar graph comparing 0.2% yield strength of the GMO-10, -10A, 21A, -21B, -22A, and -22B at 800 degrees C. No data are available for cast iron GM-2 at this temperature.

FIG. 19 is a bar graph comparing ultimate tensile strength of the GMO-10, -10A, 21A, -21B, -22A, and -22B at 800 degrees C.

FIG. 20 is a bar graph comparing total elongation of the comparison GM-2 and GMO-10, -10A, 21A, -21B, -22A, and -22B at RT.

FIG. 21 is graph showing AC₁ temperature (designated A₁) as a function of Si content in weight % of the evaluated cast irons GMO with all alloys containing a nominal 3 weight % C. The AC₁ temperature of comparison cast iron GM-2 is also shown.

FIG. 22 is a graph showing 0.2% yield strength at RT versus total phase content for comparison cast irons GM-1 and GM-2 from different casting lots and for evaluated cast irons GMO-10A, -21A, -21B, -22A, -22B, and -10.

FIG. 23 is a graph showing ultimate tensile strength at RT versus total phase content for comparison cast irons GM-1 and GM-2 from different casting lots and for evaluated cast irons GMO-10A, -21A, -21B, -22A, -22B, and -10.

FIG. 24 is a graph showing 0.2% yield strength at 600 degrees C. versus total phase content for comparison cast irons GM-1 and GM-2 from different casting lots and for evaluated cast irons GMO-10A, -21A, -21B, -22A, -22B, and -10.

FIG. 25 is a graph showing ultimate tensile strength at 600 degrees C. versus total phase content for comparison cast irons GM-1 and GM-2 from different casting lots and for evaluated cast irons GMO-10A, -21A, -21B, -22A, -22B, and -10.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides cast irons and, in particular, nodular cast irons, which have no intentional addition of Mo and which have alloy compositions to provide certain cost and use advantages over conventional Mo-bearing cast irons, in particular to increase the AC₁ temperature (onset or start of the ferrite-to-austenite transformation). Cast irons of the invention have alloy compositions wherein Si is present in a higher amount of about 3.5 weight % to about 6.0 weight % and wherein Mo is absent (i.e. substantially 0 weight % Mo) or present as an impurity element such as not exceeding about 0.01 weight % of the cast iron composition. Si preferably is about 4.5% to about 6.0% by weight and even more preferably about 4.75% to about 5.5% by weight of the cast iron composition. Mo preferably is absent from the cast iron composition.

A particular illustrative cast iron is provided consisting essentially of, in weight %, about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, and balance Fe wherein Mo is absent or present as an impurity element of the cast iron composition. C preferably is about 2.5% to about 3.5%

Another illustrative embodiment of the invention provides a nodular cast iron having a composition, in weight %, consisting essentially of about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, up to about 2.0% Cr, nodularizing agent such as Mg and/or Ce, up to about 2.0% Mn, up to about 0.75% Ni, up to about 2.5% W, up to about 2.5% Cu, up to about 1.0% V up to about 1.0% Ti, an effective amount of a nodularizing agent such as about 0.01% to about 0.10% of Mg and/or Ce, and balance Fe wherein Mo is absent or present as an impurity element. A nodular or ductile cast iron is provided when Mg, Ce or other nodularizing agent is present in the composition in an effective amount to nodularize the graphite in the as-cast microstructure.

One or more of Cu, W, V, or Ti can be present in a respective amount of about 0.25% to about 2.5% for Cu, about 0.25% to about 2.5% for W, about 0.025% to about 0.2% for V, or about 0.1% to about 0.25% for Ti of the cast iron composition.

A certain preferred Mo-free cast iron nominally consists essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% Ni, about 0.3 to about 0.5% W, an effective amount of Mg for nodularization, and balance Fe.

Another preferred Mo-free cast iron nominally consists essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% Ni, about 0.3 to about 0.5% W, about 0.4 to about 0.5% Cu, an effective amount of Mg for nodularization, and balance Fe.

Still another preferred Mo-free cast iron nominally consists essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% Ni, about 1.0 to about 1.5% W, an effective amount of Mg for nodularization, and balance Fe.

A still further preferred Mo-free cast iron nominally consists essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% % Ni, about 0.3 to about 0.5% W, about 0.4 to about 0.5% Cu, about 0.1% of each of V and/or Ti, an effective amount of Mg for nodularization, and balance Fe.

Cast irons pursuant to the invention that are free of Mo or have only impurity Mo are advantageous to provide an AC₁ temperature that is about 50 degrees C. higher than a conventional Mo-containing cast iron, although the as-cast microstructure preferably is similar in its content of graphite to that of Mo-containing cast irons. A higher AC₁ translates to a higher use or operating temperature of the cast iron. For example, cast irons of the invention can exhibit an AC₁ temperature of about 880 degrees C. and above. Cast irons of the invention can operate at temperatures of 650-700 degrees C. and above, such as 750 degrees C., encountered in exhaust manifold applications.

As mentioned above, the as-cast microstructure of cast irons pursuant to the invention preferably is similar in its content of graphite to that of Mo-containing cast irons. For example, cast irons pursuant to the invention can have about 2.75 to about 3.5 weight % graphite content, such as nominally about 3 weight % graphite content, as described and determined below.

Moreover, cast irons of the invention are advantageous to provide a higher tensile strength (e.g. about 10 ksi higher) at room temperature and 600 degrees C. than a conventional Mo-containing cast iron. For example, the cast irons of the invention can have an ultimate tensile strength above 90 ksi at 23 degrees C. (room temperature) and an ultimate tensile strength above 30 ksi at 600 degrees C. These advantages are achieved at lower alloy cost since no intentional Mo addition is made to the cast iron.

Certain low-Mo or Mo-free cast iron alloys which were evaluated in arriving at the present invention have compositions shown in Table 2 where values are weight %. These cast iron alloys designated GMO alloys were compared to Mo-containing cast iron alloys designated GM-1 and GM-2 whose compositions are shown in Table 1 where values are weight %. The evaluation and comparison initially proceeded using computational modeling to study the microstructural features and transformation temperatures for the comparison Mo-containing cast iron alloys GM-1 and GM-2 and was followed by validation of model predictions with measured transformation temperatures, microstructural phases (e.g. graphite nodule content and other phases) and mechanical properties at RT (room temperature), 600 degrees C., and 800 degrees C. from alloy casting trials. The computational modeling to study the microstructural features and transformation temperatures was effected using commercially available modeling software known as Thermocalc™ software. The software has the data base for iron base alloys and is able to predict stable phases of the alloys under equilibrium conditions as a function of temperature. The phase prediction permits knowing the transformation temperatures in the solid state, such as ferrite to austenite, and the solid to liquid state, such as melting.

TABLE 1 Chemical Analysis of Mo-Containing Cast Irons (Comparison Alloys GM-1 and GM-2) Element Low Mo (GM-1) High Mo (GM-2) C 3.29 3.35 Si 4.06 3.95 Mo 0.6 0.85 Cr 0.007 0.035 Cu 0.05 0.03 Mg 0.026 0.03 Mn 0.28 0.3 Ni 0.026 0.04 P 0.009 0.016 S 0.009 0.008 Ti — 0.006 V — 0.007 Zr — 0.0007 W — —

Comparison Mo-containing cast irons GM-1 and GM-2 are currently used in making exhaust manifolds for internal combustion engines. Computational modeling was used to compute the phase stability as a function of temperature and phase content for Mo-containing alloys in FIGS. 1 through 4. A similar example of calculated phase stability as a function of temperature and phase content for evaluated Mo-free GMO-1 cast iron alloy is shown in FIGS. 5 and 6. These figures show that the most common feature of the comparison Mo-containing cast irons (GM-1 and GM-2) is the presence of graphite, which varied from 3.25 to 3.30 wt % for the two comparison alloys. Another phase than graphite that is stable at 600, 700, and 800° C. in cast irons GM-1 and GM-2 is M₆C, where M is the metallic element(s) (see Table 3). The weight percent of M₆C at 600° C. increases from 1.1 to 1.6% with an increase in Mo content from 0.6 to 0.85 wt % in comparison cast iron GM-2 from comparison cast iron GM-2. As temperature increases to 700 and 800° C., the phase content decreases somewhat, indicating that a part of M₆C goes into solution.

TABLE 2 Chemical Analysis of Evaluated Low Mo or Zero Mo GMO Alloys Alloy C Si Mo Cr Cu Mg Mn Ni W V Ti GMO-1 3.35 4.0 — 0.03 0.05 0.03 0.3 0.03 1.7 — — GMO-2 3.35 4.0 0.2  0.03 2.0 0.03 0.03 0.3  1.0 — — GMO-3 3.2 3.75 — — 1.0 0.03 0.5 — — — — GMO-4 3.2 3.75 — — 2.0 0.03 0.5 — — — — GMO-5 2.0 6.0 — — — 0.03 0.5 — — — — GMO-6 2.0 6.0 — — 1.0 0.03 0.5 — — — — GMO-7 2.5 4.5 — — 0.5 — 0.3 0.3  — — — GMO-8 3.0 4.5 — — 1.0 — 1.0 0.5  — — — GMO-9 3.0 4.5 — 1.0  1.0 — 1.0 0.5  — — — GMO- 3.0 5.0 — 0.03 0.05 0.03 0.5 0.03 — — — 10 GMO- 2.5 5.5 — 0.03 0.05 0.03 0.05 0.03 — — — 11 GMO- 3.0 5.5 — 0.03 0.05 0.03 0.5 0.03 — — — 12 GMO- 2.0 6.0 — — 0.5 0.03 0.5 — 1.0 — — 13 GMO- 2.0 4.0 — 0.03 0.5 0.03 0.3 0.03 — — — 14 GMO- 2.0 4.5 — 0.03 0.5 0.03 0.3 0.03 — — — 15 GMO- 2.0 5.0 — 0.03 0.5 0.03 0.3 0.03 — — — 16 GMO- 2.0 5.5 — 0.03 0.5 0.03 0.3 0.03 — — — 17 GMO- 3.0 4.98 0.009 0.14 0.50 0.05 0.523 —  0.85 0.04 0.174 18 GMO- 3.0 4.98 0.009 0.14 0.80 0.05 0.523 —  0.85 0.04 0.174 19 GMO- 3.0 5.0 0.009 0.14 0.07 0.05 0.523 — 0.5 0.1 0.174 20 GMO- 3.0 5.0 0.009 0.14 0.07 0.05 0.523 — 0.5 0.1 0.174 21 GMO- 3.0 5.0 0.009 0.14 0.5 0.05 0.523 — 0.5 0.1 0.174 22

A goal of the computational modeling was to create the GMO cast iron compositions that would yield the same or similar graphite content and very similar other phase content (phases other than graphite) as in comparison cast irons GM-1 and GM-2. The calculated values of some of the new evaluated cast iron compositions are included in Table 3 where values are in weight % It can be noted from Table 3 that the compositions of GMO-1, -2, -3, and -4 alloys can produce the same graphite content as comparison cast irons GM-1 and GM-2. The GMO-2 alloy had 0.2 weight % Mo to predict its effect when W is added. This would give an idea of Mo impurity effects if Mo ever got as high as 0.2% Mo. As is shown later, this alloy GMO-2 was not deemed within the invention as a result of its lower AC₁ transformation temperature than the comparison cast irons GM-1 and GM-2.

The GMO-1 and GMO-2 alloys have similar metal carbide M₆C phases as present in comparison cast irons GM-1 and GM-2, and the total phase content for GMO-1 and GMO-2 is even higher than GM-1 and GM-2 at the temperatures evaluated. GMO-1 included metal carbide phase MC-SHP, which is mostly WC, Table 3 and FIGS. 5-6. The alloy GMO-2 having high Mo was not acceptable as a result of its lower AC₁ transformation temperature than the comparison cast irons GM-1 and GM-2. The GMO-3 and GMO-4 alloys have the same graphite content as cast irons GM-1 and GM-2 but contain a Cu-phase as opposed to M₆C. The GMO-5 and GMO-6 alloys produced lower graphite content than the comparison cast irons GM-1 and GM-2 and thus were deemed unacceptable.

Phase calculations for selected GMO alloys (from Table 2) with desired phase contents are shown in Table 4 where values are in weight %, and from this table, the following observations are worth noting: 1. The evaluated GMO compositions of Table 4 have about 3 wt % graphite. 2. GMO-10A has similar range of M₆C as comparison cast iron GM-2 but also contains other phases. The total phase content for GMO-10A is higher than GM-2 and thus can yield improved properties. 3. GMO-18 and GMO-19 are very similar and have M₆C phase similar to comparison cast iron GM-2 but also contain the M(C,N) and Cu phase. 4. GMO-21 and GMO-22 are the leaner alloy compositions (see Table 2) with M₆C (similar to comparison cast iron GM-2) but total phase content made up with the presence of M(C,N) and Cu contents.

TABLE 3 Phase Content at 600, 700, and 800° C. for GM-1 and GM-2 Alloys and Some of the Newly Designed GMO Alloys Weight Percent Phase Alloy Graphite M₆C Cu MC-SHP Content^(a) 600° C. GM-1 3.25 1.10 — — 1.10 GM-2 3.30 1.60 — — 1.60 GMO-1 3.30 2.40 — 0.15 2.55 GMO-2 3.30 1.90 1.9 — 2.80 GMO-3 3.2 — 0.9 — 0.9 GMO-4 3.2 — 1.9 — 1.9 GMO-5 2.0 — — — 0.0 GMO-6 2.0 — 0.9 — 0.9 700° C. GM-1 3.25 1.00 — — 1.00 GM-2 3.30 1.50 — — 1.50 GMO-1 3.30 1.15 — 1.00 2.25 GMO-2 3.30 1.85  1.70 — 3.55 GMO-3 3.25 — 0.7 — 0.7 GMO-4 3.2 —  1.65 — 1.65 GMO-5 2.0 — — — 0.0 GMO-6 2.0 —  0.70 — 0.75 800° C. GM-1 3.25 0.85 — — 0.85 GM-2 3.30 1.30 — — 1.30 GMO-1 3.30 0.00 — 1.75 1.75 GMO-2 3.30 1.80  1.25 — 3.05 GMO-3 3.2 — 0.3 — 0.3 GMO-4 3.2 — 1.2 — 1.2 GMO-5 2.0 — — — 0.0 GMO-6 2.0 —  0.50 — 0.50 ^(a)Total phase content other than graphite.

TABLE 4 Phase Content Summary for Selected GMO Compositions Investigated with Potential to Deliver the Desired Properties Phases (wt %) Alloy Graphite M₆C M(C, N) Cu Total^(a) 600° C. GM-2 3.30 1.10 — — 1.10 GMO-10A 2.91-3.0 1.15-1.65 0.40 0.12 1.67-2.17 GMO-18 2.91 1.40 0.38 0.47 2.25 GMO-19 2.91 1.39 0.38 0.77 2.54 GMO-20 2.91 0.907 0.49 0.12 1.517 GMO-21 2.91 0.930 0.511 0.116 1.557 GMO-22 2.90 0.950 0.52 0.477 1.947 800° C. GM-2 3.30 0.85 — — 0.85 GMO-10A 2.81-3.0 0.86-1.38 0.44-0.46 0.07 1.37-1.91 GMO-18 2.91 1.12 0.44 0.23 1.79 GMO-19 2.91 1.14 0.44 0.40 1.98 GMO-20 2.91 0.605 0.558 0.07 1.233 GMO-21 2.91 0.605 0.58 0.07 1.255 GMO-22 2.90 0.628 0.593 0.233 1.45 ^(a)Excluding graphite content.

Another goal in the development and evaluation of the GMO cast iron alloys is to increase upper use temperature in service. The maximum upper temperature of cast irons is guided by the transformation temperature (AC₁) where ferrite begins to transform to austenite (see FIG. 1). FIG. 1 also shows the temperature for completion of transformation of ferrite to austenite (AC₃) and the melting point (MP) of the alloy. The calculated values of AC₁, AC₃, and MP for the comparison cast irons GM-1 and GM-2 and all of the GMO alloys are shown in Table 5. This table shows that several of the GMO alloys can exceed the AC₁ of comparison cast iron GM-2 by about 50° C. (see GMO-5, -6, -10, -11, -12, -16, and -17). Almost all of the GMO alloys match or exceed the MP of comparison cast irons GM-1 and GM-2.

TABLE 5 Calculated Transformation Temperatures for Mo-Containing Alloys GM-1 and GM-2 and New Alloys with Low or No Mo (GMO Alloys) Temperature (° C.) Alloy AC₁ AC₃ Melting Point GM-1 853 903 1169 GM-2 854 896 1164 GMO-1 844 894 1163 GMO-2 843 893 1164 GMO-3 832 882 1180 GMO-4 832 880 1170 GMO-5 952 1048 1242 GMO-6 952 1048 1240 GMO-7 862 926 1236 GMO-8 821 900 1180 GMO-9 805 834 1180 GMO-10 896 958 1180 GMO-11 922 1000 1216 GMO-12 922 1000 1164 GMO-13^(a) GMO-14 847 900 1287 GMO-15 867 933 1273 GMO-16 893 967 1263 GMO-17 926 1007 1253 GMO-18 887 959 1169 GMO-19 882 959 1169 GMO-20 887 959 1169 GMO-21 888 959 1169 GMO-22 887 959 1169 ^(a)Calculation not performed.

Alloy Validation Trials Trial #1

Alloy GMO-10 was the first alloy subjected to the actual melt trial under foundry conditions. The compositions chosen for alloy GMO-10 are shown in Table 6. A 200-lb heat of AIM composition of GMO-10 was air-induction melted and cast into sand molds and air cooled. Half of the heat (about 100 lb) was poured into test bars and the remainder was adjusted to add 1 wt % W. This W-containing composition was called alloy GMO-10A. The charge materials that were melted comprised commercially available, powder and/or solid elemental constituents. The actual compositions achieved for alloys GMO-10 and GMO-10A are shown in Table 6 where values are weight %. This table shows that the AIM composition was met for most of the alloying elements, with exception of W, which was on the high side of the AIM value of 1%. Impurity Mo was less than 0.01 weight %. The cast bars of GMO-10 and GMO-10A had dimensions of 6½ inches length, 1⅜ inches width, and 1 inch thickness.

TABLE 6 Chemical Analysis of GMO-10 and GMO-10A Cast into Test Bars Percent GMO-10 GMO-10A Element AIM Actual AIM Actual C 3 2.93 3 2.78 Si 5 4.79 5 4.979 Mg 0.03 0.0338 0.03 0.048 Cu 0.05 0.725 0.05 0.0743 Cr 0.03 0.1374 0.03 0.135 Mn 0.5 0.525 0.5 0.523 Ni 0.03 0.0342 0.03 0.0315 W <0.01 1 1.322 P 0.0179 0.0189 S 0.008 0.007 Mo 0.0015 0.0089 V 0.0418 0.0407 Ti 0.0174 0.174 Nb <0.0020 <0.0020 Co <0.0010 <0.0010 Al 0.0168 0.0223 Ce <0.0010 <0.0010 Pb 0.0125 0.014 Sn 0.0049 0.0049

Trial #1—Transformation Temperature Measurements

Differential scanning microscopy was used to determine the transformation temperature for the comparison cast iron GM-2 and the evaluated alloys GMO-10 and GMO-10A (see FIG. 8). Measured values of transformation temperature for the three alloys are compared with the computationally predicted values in Table 7. This table shows that the agreement between the measured and calculated is excellent for the comparison Mo-containing cast iron GM-2. However, for alloy GMO-10, the measured values are higher by about 10° C. For alloy GMO-10A, which used tungsten and is not in the data base used for calculations, the measured values are 22° C. higher than the calculated values. Two observations are noted: (1) for W-containing alloy, GMO-10A, measured values differ from the calculated values by 22° C.; and (2) the measured value of AC₁ for GMO-10A is 50° C. higher than that for comparison cast iron GM-2.

Based on observations for GMO-10A, this evaluated cast iron could be considered as an alternate Mo-free cast iron to comparison cast iron GM-2.

TABLE 7 Comparison of Calculated Versus Measured Transformation Temperature from Ferrite to Austenite for GM-2, GMO-10, and GMO-10A AC₁ Transformation Temperature (° C.) Alloy Predicted Measured GM-2 854 856.9 GMO-10 879 888.8 GMO-10A^(a) 884 906.0 ^(a)Alloy shows a 50° higher transformation temperature.

Trial #1—Microstructural Analysis

The optical microstructure of cast bars of GMO-10 and GMO-10A (FIGS. 9 a, 9 b) are compared with the comparison cast irons, GM-1 and GM-2, FIGS. 10 a, 10 b. Note that the GMO alloys, especially GMO-10A [FIG. 9( b)] has microstructure very similar to that of comparison alloy GM-2 [FIG. 10( b)]. This confirms that the computationally predicted microstructures are similar to those observed.

Trial #1—Mechanical Properties

Tensile data for the comparison cast irons GM-1 and GM-2 and the evaluated cast iron alloys GMO-10 and GMO-10A are shown in FIG. 7 containing Table 8. The alloys GMO-10 and GMO-10A were also tested at 800° C. because of their higher transformation temperature (reflecting an increased upper use temperature). Tensile properties for alloys GMO-10 and GMO-10A are compared with those of comparison cast irons GM-1 and GM-2 in FIGS. 11-12, and Table 8. These figures/table show that: (1) strength values, 0.2% offset yield and ultimate tensile, for alloy GMO-10A are significantly higher at all temperatures as compared to comparison cast irons GM-1 and GM-2; and (2) the elongation values of alloy GMO-10A are lower than comparison cast irons GM-1 and GM-2 but still of an acceptable level. Tensile testing was conducted in accordance with the ASTM E-8 procedure.

Trial #2

The casting second trial was carried out on alloys GMO-21 and GMO-22. For each of these alloys, two variants were targeted (GMO-21A/GMO-21B and GMO-22A/GMO-22B). Similar to GMO-10 trial, each of the compositions was air-induction melted into 200-lb heats and split into halves, each for targeting A and B compositions. The compositions were cast into sand molds and air cooled. The charge materials that were melted comprised commercially available, powder and/or solid elemental constituents. Table 9 shows target and actual compositions in weight % achieved for alloys GMO-21A, -21B, -22A, and -22B. This table shows that for alloys GMO-21A and GMO-21B and GMO-22A, the target and actual analyses were very close. However, GMO-22B did not meet its target values of V and Ti.

Trial #2—Transformation Temperature Measurements

The transformation temperature for GMO-21A, -21B, -22A, and -22B was measured using the DSC, FIG. 13. Data from this figure and from Trial #1 are summarized in Table 10. Several points are clear from this table: (1) with the exception of data on GMO-22A, the measured values of AC₁ for GMO-21A, -21B, and -22B are about 20-30° C. higher than predicted values. It is to be noted that the predictions are based on the nominal and not actual composition and thus might change somewhat and (2) both GMO-21A and B and GMO-22B have transformation temperatures that are 50° C. higher than the values for comparison cast iron GM-2.

TABLE 9 Chemical Analysis of GMO Alloys Cast into Test Bars Percent GMO-21A GMO-21B GMO-22A GMO-22B Element Target Actual Target Actual Target Actual Target Actual C 3 2.94 3 2.88 3 3.18 3 3.26 Si 5 4.618 5 4.663 5 5.16 5 4.96 Mg 0.05 0.0434 0.05 0.0411 0.05 0.05 Cu 0.05 0.029 0.5 0.533 0.5 0.47 0.5 0.49 Cr 0.03 0.029 0.03 0.028 0.03 0.04 0.03 0.04 Mn 0.5 0.53 0.5 0.526 0.5 0.49 0.5 0.49 Ni 0.03 0.05 0.03 0.048 0.03 0.05 0.03 0.06 W 0.5 0.4784 0.5 0.474 0.3 0.32 0.3 0.29 P S Mo V 0.0023 0.0023 0.0064 0.1 0.018 Ti 0.0031 0.003 Nb 0.001 0.001 0.1 0.002 Co Al 0.0252 0.0249 Ce Pb Sn

TABLE 10 Comparison of Calculated Versus Measured Transformation Temperature from Ferrite to Austenite for GM and GMO Alloys AC1 Transformation Temperature (° C.) Alloy Predicted Measured GM-2 854^(a) 856.9 GMO-10 879   888.8 GMO-10A^(b) 884   906.0 GMO-21A^(b) 888^(c) 912.2 GMO-21B^(b) 888^(c) 902.3 GMO-22A^(d) 888^(c) 868.2 GMO-22B^(b) 888^(c) 920.8 ^(a)Predicted using actual chemistry. ^(b)Measured transformation values are ~50° C. higher for new GMO alloys as compared to the comparison GM-1 and GM-2 alloys. ^(c)Predicted using nominal chemistry. ^(d)An exception; needs to be confirmed.

Trial #2—Mechanical Properties

Tensile data for Trial #1 and Trial #2 alloys are presented in Table 11. Tensile properties, 0.2% yield strength, and ultimate tensile strength for the GMO alloys from Trial #1 and Trial #2 are compared with the comparison Mo-containing cast iron GM-2 in FIGS. 14 through 19. Comparisons with comparison cast iron GM-2 are shown for test temperatures of RT and 600° C. At 800° C., data are compared for various GMO alloys because no comparative data are available for GM-2. These figures show the following: (1) All of the GMO alloys (10, 10A, 21A, 21B, 22A, and 22B) exceed their yield strength as compared to comparison cast iron GM-2 at RT (FIG. 14) and 600° C. (FIG. 16). (2) All of the GMO alloys also exceed their ultimate tensile strength as compared to comparison cast iron GM-2 at RT (FIG. 15) and 600° C. (FIG. 17). (3) At 800° C., GMO-10A and GMO-22A show the highest value of yield and ultimate tensile strength as compared to the other GMO alloys (FIGS. 18 and 19).

Total elongation of various GMO alloys at RT is compared with comparison cast iron GM-2 in FIG. 20. The GMO alloys tend to have lower elongation than comparison cast iron GM-2. However, a value of 4 to 6% is considered acceptable from the manufacturing operations.

Measured values of the transformation temperature for comparison cast iron GM-2 and the evaluated GMO alloys are plotted as a function of Si content in FIG. 21. with the exception of one data point, the transformation temperature for cast iron compositions with about 3% C increase with increase in Si content. A 1% increase in Si content increases the transformation temperature by 60° C.

TABLE 11 Tensile Data for Mo-Free GMO Alloys 0.2% Ultimate Reduction Temperature PL Yield Tensile Elongation In Area (° C.) (Mpa) ksi Strength ksi Strength ksi (%) (%) GMO-10 23 70.8 100.7 11.0 10.0 600 38.9 46.9 16.5 14.5 800 5.9 7.0 55.0 58.7 GMO-10A 23 87.9 109.2 6.0 10.0 600 38.1 48.5 7.0 8.8 800 8.5 11.1 35.0 42.9 GMO-21A 23 74.9 93.1 5.0 10.6 600 35.6 44.4 9.0 23.3 800 6.8 8.2 32.5 53.6 GMO-21B 23 84.2 95.6 2.5 0.08 600 36.3 48.7 7.5 1.6 800 6.2 7.4 23.0 47.2 GMO-22A 23 82.4 93.3 4.0 7.8 600 27.8 32.7 8.0 3.2 800 8.5 10.2 22.5 15.8 GMO-22B 23 76.5 94.1 4.0 2.8 600 33.6 37.2 9.5 4.0 800 6.5 7.3 19.5 30.6

Yield and ultimate tensile strength data on comparison Mo-containing cast irons GM-1 and GM-2 and Mo-free alloys (GMO-10, -10A, -21A, -21B, -22A, and -22B) are plotted as a function of total phase content (other than graphite) in FIGS. 22 through 25. Data are plotted for tests conducted at RT (23° C.) for FIGS. 22-23 and 600° C. for FIGS. 24-25. The following observations are possible from these graphs: (1) Both yield and ultimate tensile strengths increase with increasing total phase content. This is true at RT and 600° C. (2) Although the trend of increasing strength values with increasing phase content is the same for both comparison cast irons GM-1, GM-2 and the GMO alloys, the whole curve for GMO alloys is shifted up from comparison cast iron GM-1 and GM-2 by nearly 10 ksi. It is believed that such a shift is related to an increase in solid-solution strength of the new alloys, which is most likely caused by their higher silicon content as opposed to comparison Mo-containing cast irons GM-1 and GM-2, although applicants do not wish or intend to be bound by any theory in this regard.

Although certain detailed embodiments of the invention are disclosed herein, those skilled in the art will appreciate that the invention is not limited to these embodiments but only as set forth in the appended claims. 

1. A cast iron wherein Si is present in an amount of about 3.5 weight % to about 6.0 weight % and wherein Mo is absent or present as an impurity element.
 2. The cast iron of claim 1 wherein Si is about 4.5% to about 6.0%.
 3. The cast iron of claim 2 wherein Si is about 4.75% to about 5.5%.
 4. The cast iron of claim 1 which a nodular cast iron.
 5. A cast iron that consists essentially of, in weight %, about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, and balance Fe wherein Mo is absent or present as an impurity element.
 6. The cast iron of claim 5 wherein Mo does not exceed about 0.01 weight % of the cast iron.
 7. The cast iron of claim 5 wherein Si is about 4.5% to about 6.0%.
 8. The cast iron of claim 7 wherein Si is about 4.75% to about 5.5%.
 9. The cast iron of claim 5 wherein C is about 2.5% to about 3.5%.
 10. The cast iron of claim 5 having an AC₁ temperature of about 880 degrees C. and above.
 11. The cast iron of claim 5 including Mg and/or Ce as a nodularizing agent.
 12. The cast iron of claim 5 having an ultimate tensile strength above about 90 ksi at 23 degrees C.
 13. The cast iron of claim 12 having an ultimate tensile strength above about 30 ksi at 600 degrees C.
 14. A cast iron having a composition, in weight %, consisting essentially of about 2.0% to about 4.0% C, about 3.5% to about 6.0% Si, up to about 2.0% Cr, up to about 2.0% Mn, up to about 0.75% Ni, up to about 2.5% W, up to about 2.5% Cu, up to about 1.0% V up to about 1.0% Ti, effective amount of a nodularizing agent, and balance Fe wherein Mo is absent or present as an impurity element.
 15. The cast iron of claim 14 wherein Mo does not exceed about 0.01 weight % of the cast iron.
 16. The cast iron of claim 14 including about 0.01% to about 0.10% Mg as a nodularizing agent.
 17. The cast iron of claim 14 wherein Si is about 4.5% to about 6.0%.
 18. The cast iron of claim 17 wherein Si is about 4.75% to about 5.5%.
 19. The cast iron of claim 14 having one or more of Cu, W, V, or Ti present in a respective amount of about 0.25% to about 2.5% for Cu, about 0.25% to about 2.5% for W, about 0.025% to about 0.2% for V, or about 0.1% to about 0.25% for Ti.
 20. The cast iron of claim 14 having an AC₁ temperature of about 880 degrees C. and above.
 21. The cast iron of claim 14 having nominally about 3 weight % graphite content in its cast microstructure.
 22. The cast iron of claim 14 having an ultimate tensile strength above 90 ksi at 23 degrees C.
 23. The cast iron of claim 22 having an ultimate tensile strength above about 30 ksi at 600 degrees C.
 24. A Mo-free cast iron nominally consisting essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% Ni, about 0.3 to about 0.5% W, an effective amount of Mg for nodularization, and balance Fe.
 25. A Mo-free cast iron nominally consisting essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% Ni, about 0.3 to about 0.5% W, about 0.4 to about 0.5% Cu, an effective amount of Mg for nodularization, and balance Fe.
 26. A Mo-free cast iron nominally consisting essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.03 to about 0.05% Mg, about 0.5% Mn, about 0.03% Ni, about 1.0 to about 1.5% W, an effective amount of Mg for nodularization, and balance Fe.
 27. A Mo-free cast iron nominally consisting essentially of, in weight %, about 3.0% C, about 5.0% Si, about 0.03% Cr, about 0.5% Mn, about 0.03% % Ni, about 0.3 to about 0.5% W, about 0.4 to about 0.5% Cu, about 0.1% of each of V and/or Ti, an effective amount of Mg for nodularization, and balance Fe.
 28. A component of an internal combustion engine made of the cast iron of claim
 1. 29. The component of claim 28 which is an exhaust manifold. 